indeed.
the concept of a negative number implies an existing subtraction.
0 minus 9 equals negative 9
so any negative number in any equation can be thought of as (0 minus positive number)
so 9 - (0 - 9) = 9 - 0 - -9 = 9 - 0 + 9 = 9 + 9 = 18.
logically, it's the same concept as in grammar; a double negative cancels each negative out.
"this statement is not not true"